3.11
Carrier-Mediated Transport
Carrier-mediated transport is a pivotal process in drug absorption, particularly for lipid-insoluble drugs, and encompasses facilitated diffusion and…
Carrier-mediated transport facilitates the movement of lipid-insoluble drugs via membrane-spanning transporters.
As drug concentration increases during transport, absorption rates initially rise but eventually plateau because carrier proteins become saturated.
Carrier-mediated transport has two subtypes: facilitated diffusion and active transport.
Facilitated diffusion involves drug transport along their concentration gradient without using energy.
In contrast, active transport requires ATP to transport drugs against their concentration gradient.
There are two types of transporters. Uptake transporters, like amino acid transporters, move drugs like methyldopa and gabapentin into the blood, increasing their absorption.
Efflux transporters, such as P-glycoprotein, remove drugs back to the GI lumen, lowering their absorption.
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Q1: What is carrier-mediated transport and how does it differ from passive diffusion?
Carrier-mediated transport facilitates movement of lipid-insoluble drugs through membrane-spanning transporters, enabling absorption of drugs that cannot cross membranes passively. Unlike passive diffusion, this process involves specific protein carriers and can occur via facilitated diffusion (along concentration gradients without energy) or active transport (against gradients using ATP). This mechanism is essential for drugs like methyldopa and gabapentin.
Q2: Why does drug absorption plateau during carrier-mediated transport?
Absorption rates plateau because carrier proteins become saturated as drug concentration increases. Once all available transporters are occupied, the system reaches maximum capacity and cannot transport additional drug molecules, regardless of further concentration increases. This saturation point represents the transporter's maximum velocity and is a key characteristic distinguishing carrier-mediated from passive transport mechanisms.
Q3: What are uptake transporters and how do they enhance drug absorption?
Uptake transporters move drugs into the blood across intestinal epithelial cells, increasing absorption. Located at brush borders and basolateral membranes, these transporters facilitate movement of amino acids, peptides, and other substances. The intestinal oligopeptide transporter PepT1 exemplifies this function, enabling absorption of di- and tripeptides formed from digested oral proteins, thereby enhancing peptide drug bioavailability.
Q4: How do efflux transporters like P-glycoprotein reduce drug absorption?
Efflux transporters such as P-glycoprotein (MDR1) pump drugs back into the gastrointestinal lumen, decreasing intestinal absorption. These ATP-binding cassette proteins are strategically positioned to protect the body from undesirable compounds and xenobiotics. P-glycoprotein's widespread expression across the GI tract and other tissues, particularly the adrenal medulla, enables its defensive role in reducing permeability of lipophilic and cytotoxic drugs from lumen to blood.
Q5: What is the difference between facilitated diffusion and active transport in drug absorption?
Facilitated diffusion moves drugs along their concentration gradient without energy expenditure, relying solely on transporter proteins to facilitate movement. Active transport, conversely, requires ATP to transport drugs against their concentration gradient, enabling absorption of drugs that would otherwise accumulate outside cells. Both are carrier-mediated processes but differ fundamentally in energy requirements and directional capability.
Q6: How do organic anion transporters contribute to drug absorption?
Organic anion transporters driven by α-ketoglutarate facilitate movement of drugs like pravastatin and atorvastatin into the blood, enhancing their absorption. These uptake transporters recognize specific drug structures and actively transport them across intestinal epithelial cells. Their presence demonstrates how diverse transporter types work together to optimize absorption of lipid-insoluble drugs with different chemical properties.
Q7: What role does P-glycoprotein play in drug resistance and cellular protection?
P-glycoprotein contributes to treatment resistance by pumping drugs out of cells, reducing intracellular drug concentration and therapeutic efficacy. This efflux transporter protects vital organs and cells from xenobiotics and cytotoxic compounds through its widespread expression across tissues. In cancer cells, elevated P-glycoprotein expression is a major mechanism of multidrug resistance, making it a critical target for overcoming treatment failure.